feat(alter-page): INSERT INTO container — append widgets as a container's children

[ako]

Summary

Adds ALTER PAGE … { insert into <container> { … } } — appending widgets as a container's children. Previously ALTER PAGE could only INSERT BEFORE/AFTER a sibling widget, so there was no way to fill an empty container or add a widget to a container/dataview without an existing sibling to anchor to.

alter page Mod.Home {
  insert into ctnToolbar {                 -- fills an EMPTY container
    actionbutton btnNew (caption: 'New', action: nothing, buttonstyle: primary)
  }
}
alter page Mod.Detail {
  insert into dv {                          -- children take the dataview's entity
    dynamictext dtTitle (content: '{1}', contentparams: [{1} = Title], rendermode: H1)
  }
}

What's included (full-stack)

• Grammar — INSERT INTO widgetRef { pageBodyV3 } (regenerated parser; generated files not committed).
• AST / visitor — InsertWidgetOp.Position = "INTO".
• Executor — for INTO, the new children take the target's own entity context (e.g. a dataview's entity), not its enclosing container's.
• Mutators — both the MPR-file pagemutator and the live-Studio-Pro mcp mutator append to the container's Widgets list. An empty container omits that list, so it's created with the Mendix widget-list marker and the (grown) bson.D is written back into its parent.
• Scope — simple containers (DivContainer, Container, DataView, GroupBox, ScrollContainerRegion, Section). LayoutGrid (Rows/Columns) and TabContainer (TabPages) have no single child list and return a clear error pointing to INSERT BEFORE/AFTER a widget inside the target column/tab.

Testing

• Verified on Mendix 11.12.1 (mx check 0 errors): insert into an empty container, append to a non-empty one, and insert a data-bound widget into a dataview.
• Non-container target (e.g. a DynamicText) gives a clear error.
• Unit test for the mutator INTO path + error case (page_mutator_test.go); doctype example (mdl-examples/doctype-tests/alter-page-insert-into.mdl); full mdl/... + syntax test packages green.
• Docs: alter-page skill, MDL_QUICK_REFERENCE.md, and mxcli syntax help (page + snippet) updated.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JXnEgoc2NQP1Y2TWMCMXC4

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